I remembered when my brother and I were young we would run around the house finding different ingredients of our own to make up these silly concoctions, that if our Mom ever knew what we were doing we'd probably still be grounded today. One time a mixture of mine exploded and hit the ceiling leaving a big brown stain, I was creative enough to figure out a little plaster of paris would fix that mess.
I now set my son up with his science kit, but we also use other small containers and cups as well. I make sure to put towels under everything because i know he will spill. I usually find 3 edible things in case he decides to taste his experiment when I'm not looking. Juice, soda, milk, soup any left overs that we haven't eaten in a couple days; these are some of the things I give him in separate cups so that he can pour them from cup to cup experimenting. It's also a great way to use up some unwanted candy around the house:) My favorite experiment was pickle juice and jelly beans, it was quite fun but tasted horrible.